Although the utilization of coal to produce mechanical and electrical energy through combustion provides a fundamental contribution to our energy needs, this technology suffers certain limitations. These limitations arise through concern about the environmental effects of combustion, through inefficiencies and excessive costs involved in transporting and burning certain coals, and because the universal technique of steam generation imposes a localized utilization of steam at the time of generation. These considerations have stimulated much effort to develop processes for the conversion of coals into fuel gas, a storable, readily transported form of fuel. Gasification of coal can overcome or mitigate many of the objections raised, but current gasification technology is, in turn, not free of problems. Paramount is the enormous capital cost projected for conventional coal gasification plants. This cost makes the product gas expensive. The expense is a reflection of the cost of massive high temperature chemical reaction vessels capable of withstanding high pressures and the cost of catalytic reactors susceptible to contaminant poisoning. Accordingly, the present state of the art of thermal gasification of coal leaves considerable room for improvement.
On the other hand, many useful products have been obtained by the action of microbial organisms in the digestion of carbohydrates, such products including ethanol, insulin by genetically altered microorganisms, and methane from the anaerobic digestion of biomass. Because of the slow growth or action of the microbes, it is often necessary to use a large reactor to produce biochemical reaction products in commercially significant quantities. Often, startup times ranging up to several months are required for the microbes to grow to achieve a sufficient population density in the substrate to produce usable quantities of the desired biochemical products. Further, conditions for the microorganisms, including temperature, pH and toxic substance concentrations, must be maintained within critical limits to avoid killing the microbes. Thus, if such conditions are not maintained, mortality of the microorganisms will result and their growth must be reinitiated.
In the conversion of substrates to methane in commercially significant quantities, the above problems are particularly acute because of the large volumes of substrate and methane involved. Much effort has been directed to providing suitably economic reactors for the conversion of various substrates into methane. For example, U.S. Pat. No. 4,356,269 describes a semisubmerged insulated apparatus which has a preheating chamber provided with a heating device, a gas processing chamber in which the microorganisms are grown, and a storage chamber for spent manure.
The anaerobic digestion of a substrate is typically a three-step process in which complex organic materials are converted to the end products of methane and carbon dioxide. In the initial steps, complex organic molecules are converted into substances such as propionate, butyrate, valerate, lactate, formate and ethanol, and eventually into acetate. The organisms responsible for this conversion are collectively termed acid formers and may be either anaerobic or facultative in nature. The final step, conversion of acetate to methane and carbon dioxide, is performed by organisms collectively termed methane formers, or methanogens, which are strictly anaerobic. Because the methanogens generally grow more slowly than do the acid formers, the final step of the process is generally considered the rate limiting step. Generally, conversion of a complex organic substrate yields a gas which is typically fifty to seventy percent methane and thirty to fifty percent carbon dioxide.
In the biogasification of coal and other substrates containing macromolecular substances, however, the substrate is not readily amenable to digestion by acid formers and methanogens. Acid formers are generally unable to convert the high molecular weight substances, especially those containing fused aromatics such as coal, for example, into the lower molecular weight acids required by the methanogens. Thus the development of technology for the biogasification of coal and other macromolecular substances requires an acceptable means of treating such substrates prior to the more familiar digestion with acid formers and methanogens used in the gasification of other less complex substrates.
It has been reported to subject coal to alkaline hydrolysis in order to break down the physical and chemical structure of the coal to make it more accessible to microbial action, for example, to upgrade the coal by biological removal of nitrogen and sulfur contaminants. For instance, it was reported that milled subbituminous coal was subjected to alkaline hydrolysis at 200-300.degree. C. for 30-60 minutes with 0-20% sodium carbonate on a coal volatile solids basis in "Alkaline Hydrolysis Transformation of Coal," Electric Power Research Institute Report EPRI AP-4585, Research Project 2655-2 (May 1986). Such alkaline hydrolysis of coal is also disclosed in aforementioned patent applications, U.S. Ser. Nos. 693,028 and 816,289, which are hereby incorporated herein by reference. Also, disclosed in said applications are reactors, conditions and microorganisms for the biogasification of coal such as lignite and the alkaline hydrolysis products thereof.
It has also been reported that some aerobic microorganisms may degrade lignin structures in coal. However, it has been shown that such organisms require a very controlled diffusion of oxygen, and it is difficult to maintain the coal at the proper conditions for such degradation.
It has also been known that symbiotic microorganisms inhabit the digestive system of some higher organisms and aid in the digestive process. For example, it is well-known that termites can dissociate wood, which consists primarily of lignin, hemicellulose, and cellulose. Anaerobic degradation of wood by termites has been attributed to the symbiotic microorganisms which inhabit the termite's digestive system and which are apparently crucial for the insects' survival. The digestive tract of termites consists of three major sections: The foregut, the midgut, and the hindgut [Noirot, C. and C. Noirot-Timothee, "The Digestive System," in K. Krishna and E. M. Weesive (Eds.), Biology of Termites, pp. 48-88 (Academic Press, N.Y., N.Y. 1969)]. It has been suggested that termites "pretreat" wood in their foregut and then convert the pretreated substrate to food in their hindgut. The bulk of the symbiotic microbiota are contained in the hindgut. The pretreatment may be accomplished by enzymes produced by the microorganisms, at least one of which serves to break down the lignin to smaller molecules. The class of enzymes which dissimilate lignin are generally known as "ligninases".
Up to 83% of the wood-lignin, 99% of the wood-cellulose, and 93% of the wood-hemicellulose has been reported to be degraded by termites in Wood, T. G., "Food and Feeding Habits of Termites," in Production Ecology of Ants and Termites, M. V. Brian (ed) pp. 55-8 (Cambridge Univ. Press., Cambridge, United Kingdom 1978). The extent of lignin decomposition varies widely among termites as reported therein:
______________________________________ % Lignin Termite Food Degraded ______________________________________ Lower Termites: Calotermes sp. wood 2-26 Heterotermes sp. " 14-40 Reticulitermes sp. " 70-83 Hodotermes sp. red grass 0.3 Higher Termites: Nasutitermes s. wood 42-82 ______________________________________
Although the majority of termites feed on wood, some species having a decided preference for leaves, grass, humus, and dung have been reported in McMahan, E., Feeding Relationships in the Biology of Termites, K. Drishna and F. Weesner (eds.) pp. 387-406 (Academic Press, New York, N.Y. 1969) and Lee, E. and T. G. Wood, Termites and Soils (Academic Press London 1971). All of these foods are rich in lignin, hemicellulose, and cellulose.